It is known in the art to which this invention pertains that plastic parts prior to electroless plating and subsequent electroplating are pre-treated by a sequence of steps basically including etching the surface of the plastic with an aqueous acid solution containing hexavalent chromium ions, one or more water rinses, neutralizing with a dilute inorganic acid such as hydrochloric acid, further water rinses, contacting the surface of the substrate with an acid tin-palladium complex, and additional water rinse, accelerating the activated surface of the plastic, and then one or more water rinses prior to the electroless nickel plating.
After etching acrylonitrile-butadiene-styrene [ABS], polyaryl ethers, polyphenylene oxide, nylon or any other usually chemically plateable plastics employing a mixture of hexavalent chromium and sulfuric acid, it is highly desirable that there be removed essentially all traces of hexavalent chromium ions from the micropores of the plastic surface, as well as from difficult to reach areas such as blind holes, recesses and regions behind the rack contacts. In the prior art, a two or three stage rinsing cycle is frequently used to remove as much hexavalent chromium as is possible. As is also known, any hexavalent chromium carried into the activator step can react with the stannous chloride present in the activator by an oxidation reduction reaction. The valuable stannous chloride which stabilizes the activator solution is thereby lost by this redox reaction. It also occurs that some hexavalent chromium may survive immersion in the activator step and yet leak out during subsequent rinsing or processing steps. In this event, palladium adsorbed on the polymeric parts is removed in the event that drops of hexavalent chromium solution fall onto the other parts or bleed through the blind holes if present. The palladium deficient areas will appear as misplates or skips after subsequent electroplating. If contact areas are effected, complete burn-off may occur.
In order to eliminate the possibility of contamination of the subsequent steps in the process by hexavalent chromium ions, a neutralizer or chromium reducing step is conventionally utilized after water rinsing of the etched polymeric parts prior to the activation of the surfaces thereof. A reducing agent known to the art is NaHSO.sub.3 or SO.sub.2 at a pH of about 3.0 which generally rapidly reduces the hexavalent chromium upon contact with NaHSO.sub.3. One difficulty, however, with SO.sub.2 reducing agents is that they must be thoroughly rinsed to prevent carry-over into the activator, where they may reduce and decompose the palladium-tin complexes. Hydrazine compounds are also effective reducing agents for hexavalent chromium ions, but their concentration must be carefully controlled or the plastisol coatings on the racks will become sensitized and plating in electroless nickel will begin. Stannous chloride is another effective reducing agent for use in neutralizers, and stannous chloride can be dragged into the activator without harm. Never-the-less, this compound is sensitive to air oxidation, and air agitation is a very useful method of agitating neutralizers to improve contact and aid in the removal and reduction of hexavalent chromium ions from blind holes and rack contacts.
Relatively small amounts of hexavalent chromium contamination of the order of 10 ppm in an acidic accelerator results in rather severe skipping on plastic parts processed through the system. There are a number of reducing agents which possess the capability of reducing chromium to the relatively harmless trivalent state. For example, SnCl.sub.2 ; NaH.sub.2 PO.sub.2.H.sub.2 O; hydrazine; and NaHSO.sub.3. The rate of reaction of NaH.sub.2 PO.sub.2.H.sub.2 O is very slow, however, and hydrazine or NaHSO.sub.3 are such powerful reducing agents that they reduce not only the hexavalent chromium ions, but also other metal ions, such as Ni.sup.+.sup.2 and/or Pd.sup.+.sup.2 which have a beneficial effect in the acidic accelerator solutions. Accordingly, the problem faced is to select a reducing agent which is speedy and powerful enough to render the chromium contamination harmless, while not interfering with the action of beneficial metal ions which have been purposely introduced into the system.
The problems of hexavalent chromium contamination in the acidic accelerator, as well as in the other solutions in the process cycle, are significantly increased if there is not effected a speedy reduction of the chromium in the neutralizer step. Here again, the selection of reducing agents is restricted by considerations of compatibility with the other solutions in the cycle, and also by the need to avoid plating on the rack coatings.